gdb/riscv: Write 4-byte nop to dummy code region before inferior calls

diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 7f3a1f6cbc3..817e76a423d 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -1621,11 +1621,44 @@  riscv_push_dummy_code (struct gdbarch *gdbarch, CORE_ADDR sp,
 		       struct type *value_type, CORE_ADDR *real_pc,
 		       CORE_ADDR *bp_addr, struct regcache *regcache)
 {
+  /* A nop instruction is 'add x0, x0, 0'.  */
+  static gdb_byte nop_insn[] = { 0x13, 0x00, 0x00, 0x00 };
+
   /* Allocate space for a breakpoint, and keep the stack correctly
-     aligned.  */
+     aligned.  The space allocated here must be at least big enough to
+     accommodate the NOP_INSN defined above.  */
   sp -= 16;
   *bp_addr = sp;
   *real_pc = funaddr;
+
+  /* When we insert a breakpoint we select whether to use a compressed
+     breakpoint or not based on the existing contents of the memory.
+
+     If the breakpoint is being placed onto the stack as part of setting up
+     for an inferior call from GDB, then the existing stack contents may
+     randomly appear to be a compressed instruction, causing GDB to insert
+     a compressed breakpoint.  If this happens on a target that does not
+     support compressed instructions then this could cause problems.
+
+     To prevent this issue we write an uncompressed nop onto the stack at
+     the location where the breakpoint will be inserted.  In this way we
+     ensure that we always use an uncompressed breakpoint, which should
+     work on all targets.
+
+     We call TARGET_WRITE_MEMORY here so that if the write fails we don't
+     throw an exception.  Instead we ignore the error and move on.  The
+     assumption is that either GDB will error later when actually trying to
+     insert a software breakpoint, or GDB will use hardware breakpoints and
+     there will be no need to write to memory later.  */
+  int status = target_write_memory (*bp_addr, nop_insn, sizeof (nop_insn));
+
+  if (riscv_debug_breakpoints || riscv_debug_infcall)
+    fprintf_unfiltered (gdb_stdlog,
+			"Writing %lld-byte nop instruction to %s: %s\n",
+			((unsigned long long) sizeof (nop_insn)),
+			paddress (gdbarch, *bp_addr),
+			(status == 0 ? "success" : "failed"));
+
   return sp;
 }
 
diff --git a/gdb/testsuite/gdb.arch/riscv-bp-infcall.c b/gdb/testsuite/gdb.arch/riscv-bp-infcall.c
new file mode 100644
index 00000000000..9bd40af7eed
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/riscv-bp-infcall.c
@@ -0,0 +1,29 @@ 
+/* This file is part of GDB, the GNU debugger.
+
+   Copyright 2019 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+void
+dummy_call ()
+{
+  asm ("" ::: "memory");
+}
+
+int
+main ()
+{
+  dummy_call ();
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.arch/riscv-bp-infcall.exp b/gdb/testsuite/gdb.arch/riscv-bp-infcall.exp
new file mode 100644
index 00000000000..6d41c477a13
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/riscv-bp-infcall.exp
@@ -0,0 +1,56 @@ 
+# Copyright 2019 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test GDB for RISC-V always uses an uncompressed breakpoint when
+# setting up for an inferior call.
+
+if {![istarget "riscv*-*-*"]} {
+    verbose "Skipping ${gdb_test_file_name}."
+    return
+}
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+if ![runto_main] then {
+   fail "can't run to main"
+   return 0
+}
+
+# Figure out where the breakpoint will be placed taking account for
+# stack alignment, and alloction of the dummy code area.
+set bp_addr [get_valueof "/x" "\$sp" 0]
+set bp_addr [format 0x%x [expr ($bp_addr & ~0xf) - 0x20]]
+
+# Fill the region we know will be used as the scratch area with the
+# compressed nop instruction.  If GDB fails to overwrite this with an
+# uncompressed nop then a compressed breakpoint will be used in the
+# following inferior call.
+for {set i 0} {$i < 16} {incr i 2} {
+    gdb_test_no_output "set *((unsigned short *) (${bp_addr} + $i))=0x1" \
+	"place compressed nop in scratch area at offset $i"
+}
+
+# Make an inferior call.  GDB should write an uncompressed nop into
+# the scratch area and so force the use of an uncompressed breakpoint,
+# however, if this mechanism fails and GDB uses a compressed
+# breakpoint, and the target doesn't support compressed instructions,
+# then we would expect weird things to happen here.
+gdb_test_no_output "set debug riscv breakpoints 1"
+gdb_test "call dummy_call ()" \
+    ".*Using EBREAK for breakpoint at $bp_addr \\(instruction length 4\\).*"